In many applications, substantially planar work pieces, such as e.g. metal sheets, need to be removed one by one from a stack of work pieces. Corresponding methods and devices allowing for automatic removal and feeding e.g. to a processing station, are known. Often, these devices seize the top work piece from the stack by suitable grippers, e.g. magnetic elements or suction cups, and transport it to a desired location by moving the grippers together with the seized work piece.
One of the applications where corresponding devices are used is the processing of metal sheets in order to manufacture car body parts. Large-sized metal sheets are punched and shaped to obtain the desired geometry, in particular in a multi-stage press or press line. Prior to processing, a feeder seizes the top work piece of a stack and feeds it to the first processing station, which may be the head press but also stations for pre-processing such as de-greasing, cleaning, etc.
Removing the top sheet—and only the top sheet—from the stack is difficult: Often, neighboring sheets adhere to each other, e.g. because of coatings of the sheets (such as an oil film), which leads to sticking together of two or more sheets. However, if more than one sheet is seized and fed to the downstream process line, this may lead to severe damage up to destruction of a press tool. Current demands regarding processing speed make de-stacking even more challenging.
Several methods and devices for automatic destacking are known from the prior art. As an example, German patent No. 671 402 (Schuler) relates to an automatic apparatus for lifting and feeding strips to high speed presses, punching machines or similar. The strips are lifted from a stack and fed to a pincing feeder. A stop for the strip is arranged below the pincer neighboring the stack. Between the stop and the downstream edge of the stack, a lifting device is arranged for lifting the downstream end of the strip to the transport plane of the pincer. A suction device is arranged close to the upstream end of the stack. It is used for partially detaching the strip from the stack and may also be used for feeding the strip to the pincing feeder. Alternatively, there may be a separate feeding device for that purpose.
DE 33 12 459 A1 (Lewecke Maschinenbau) relates to a method for separating planar work pieces, such as flake boards, adhering to each other, when the topmost work piece is to be lifted by a suction gripper. For that purpose, pressurized air is directed into the gaps between adjacent workpieces. Furthermore, the separation process is supported by jerky accelerations and decelerations of the lifting movement.
DE 10 2005 062 713 A1 (Holzma Plattenaufteiltechnik) discloses a device for separating and removing a partial stack comprising at least one extended sheet from a remaining stack. The device includes an air nozzle which may be directed to the region of a separating plane below the lowest sheet of the partial stack. It may further include a separating element which may be introduced into a gap produced by the action of the air nozzle, in order to mechanically securing the gap and enlarging it, if required. In a further step, the partial stack is removed by means of a transport device comprising a transport bar and tongs.
Still, the known devices have drawbacks when removing extended and comparably heavy sheets, especially when a high throughput is required. The additional steps for improving the separation make the removal process more complicated and time-consuming.